There is a need to detect and measure the presence of volatile compounds in natural and artificially contained sources of water, and in other aqueous fluids. Many such compounds are classed as volatile organic compounds, VOC's, and include irritants, oestrogens, carcinogens and other chemicals harmful to humans, animals and plant life. They are a matter of concern in marine environments, waterways, harbours, industrial process water, and waste water. They may also be of significance in other aqueous fluids such as in the detection of alcohols due to fermentation of liquid food stuffs and in brewing.
It is frequently of interest to search for the presence of gaseous species within large expanses of water, and to trace their source and extent in near real time, as for example may arise from spillage of petrochemicals into sea water. Such bodies of water are also subject to currents and drift. Therefore it is preferable for sensors which are able to detect target analytes in water to provide a fast and quantitative measurement of their concentration.
It should be noted that clean potable water contains very low or imperceptible concentrations of any VOC's, whilst polluted waters may contain any of a wide suite of VOC's. Therefore VOC in water detectors may be of most service in rapidly identifying the presence of a suite of different analytes in water, so as to identify the source of VOC's confidently before they disperse, and enable a select few samples of water to be retrieved for less time critical and more costly detailed analysis. A detector used in this way is commonly known as a screening tool.
One class of such sensors or detectors contain a membrane, typically hydrophobic, providing a barrier between the aqueous sensed environment and a detector enclosure, through which the analyte is able to diffuse. In GB Patent Application GB1421306.0, which is incorporated herein by reference, one such detector, having as one member a gaseous enclosure is located within a few millimeters of a preferably hydrophobic and porous membrane separating it from water prospectively containing VOC's. The detector is claimed to provide a response to VOC's in water at some gas equilibrated concentration.
The present invention concerns the operation and configuration of a photoionisation detector for the detection of volatiles in water.
One problem that can be presented to sensors of volatile in water sensors such as in GB1421306.0 is that of condensation. A volatile in water sensor may be plunged from a relatively cold air or watery environment into water which is significantly warmer. In such circumstances water vapour in gas at the liquid interface of the sensor membrane is warm and at a saturated vapour concentration. On free diffusion through a porous membrane into the cold sensor interior, the water laden air is cooled to a temperature below its dew point, causing water condensation within the sensor. After many fluctuations in temperature the water in the sensor gaseous cavity may accumulate to such an extent as to cause the sensor to fail.
The present invention seeks to mitigate the above mentioned problem and therein extend the time over which a volatile in water sensor can operate within water.